Fibre optic sensing is a known technique where an optical fibre, deployed as a sensing fibre, is interrogated with interrogating radiation and radiation which emerges from the fibre is detected and analysed to determine environmental changes acting on the optical fibre. Some fibre optic sensors rely on deliberately introduced features within the fibre, e.g. fibre Bragg gratings or the like, to induce reflection from a point in the fibre. In a fibre optic distributed sensor however the radiation which is backscattered from inherent scattering sites within the fibre is detected. The sensing function is thus distributed throughout the fibre and the spatial resolution and arrangement of the various sensing portions depends on the characteristics of the interrogating radiation and the processing applied.
Fibre optic sensors for distributed temperature sensing (DTS) are known which rely on detecting light which has been subjected to Brillouin and/or Raman scattering. By looking at the characteristics of the Brillouin frequency shift and/or the amplitudes of the Stokes/anti Stokes components the absolute temperature of a given sensing portion of fibre can be determined. DTS is a useful technique with a range of applications but most DTS systems require relatively long time averages to provide the desired accuracy, meaning such DTS systems are less useful for detecting relatively rapid changes in temperature.
Fibre optic sensors for distributed acoustic sensing (DAS) are also known. Various types of DAS sensor have been demonstrated including sensors based on Rayleigh scattering of light from the sensing fibre. Light transmitted into an optical fibre will be scattered from the various inherent scattering sites within an optical fibre. A mechanical vibration of the fibre, such as caused by an incident acoustic wave, will alter the distribution of scattering sites resulting in a detectable change in the properties of the Raleigh backscattered light. Analysing such changes allows relatively high frequency vibrations/acoustic stimuli acting on sensing portions of the optical fibre to be detected.
DAS sensors therefore perform repeated interrogations of the sensing fibre. Each interrogation involves transmitting at least one pulse of coherent optical radiation into the optical fibre and detecting the intensity of backscattered light from each of a number of sensing portions of the sensing fibre, also called channels, of the DAS sensor. In one type of DAS sensor the intensity of backscatter from a given channel in response to separate interrogations of the sensing fibre is monitored to determine any acoustic stimulus acting on the fibre. Typically each interrogation involves launching a single continuous pulse of interrogating radiation. In the absence of any environmental stimulus the backscatter intensity from any given sensing portion should remain the same for each repeated interrogation (provided the characteristics of the interrogating pulse remains the same). However an environmental stimulus acting on the relevant sensing portion of the fibre will result in an optical path length change for that section of fibre, e.g. through stretching of the relevant section of fibre and/or a refractive index modulation. As the backscatter from the various scattering sites within the sensing portion of fibre will interfere to produce the resulting intensity, a change in optical path length will vary the degree of interference and thus result in a change in backscatter intensity. This change in intensity can be detected and used as an indication of a disturbance acting on the fibre, such as an incident acoustic wave.
Such DAS sensors, in which the measurement signal is based on intensity variations in the detected Rayleigh backscatter, have been advantageously employed in a wide range of applications. One issue with such sensors however is that the relative intensity change in response to a given input stimulus will vary from channel to channel and can also vary for a given channel over time. In other words the gain of the channels is variable. This means that it can be difficult to determine quantitative information about the stimulus from such a sensor. Also such sensors typically do not provide any reliable detection of low frequency disturbances on the optical fibre, i.e. the level of low frequency signal that can detected without signal distortion is low.
In another type of DAS sensor each interrogation involves launching two pulses of differing optical frequencies into the fibre. This means that the backscatter received at the detector comprises backscatter from both pulses, which will interfere, and thus there will be a signal component at the frequency difference between the pulses. If the two pulses are spatially separated in the fibre then any environmental disturbance acting on the fibre, between portions of the fibre where the pulses are reflected from, will lead to an optical path length change. This in turn will produce a phase change in the signal at this difference frequency, which can be thought of as a signal at a carrier frequency. By an appropriate choice of carrier frequency and processing of the detected signal this phase change can be related to the amplitude of the disturbance acting on the fibre. Again the characteristics of the interrogating radiation, i.e. the frequencies and durations of the two pulses, would typically be the same for each interrogation.
Such a phase based DAS system can provide an indication of the actual amount of phase shift caused by an incident stimulus and thus provide a quantitative measure of amplitude of any disturbance.
Typically DAS sensors have been used to detect relatively fast changing dynamic stimuli acting on the sensing fibre, i.e. to detect stimuli with frequencies of the order of tens of Hz or higher. The signals detected by each channel of the DAS sensor may therefore typically be high pass filtered as part of the processing to remove any low frequency effects and DC offset.
Recently however it has been proposed to detect and monitor low frequency stimuli acting on the sensing fibre using Rayleigh based DAS sensing. Low frequency effects, such as low frequency strains and/or temperature changes can result in optical path length changes in the sensing portions of the optical fibre for instance through physical length changes of the fibre and/or refractive index modulation. Such effects have typically previously been discounted as low frequency noise and, as mentioned, reduced/removed by filtering. However it has been appreciated that the low frequency effects in the Rayleigh backscatter may provide useful information about temperature and strain changes.
One issue however with Rayleigh based DAS sensors is frequency drift in the optical source which is used to generate the coherent interrogating radiation, e.g. laser phase noise. DAS sensors typically use a laser as an optical source in a DAS interrogator unit. The output of the optical source is typically input to one or more amplitude and/or frequency modulators to generate the waveform of the desired interrogating pulses, e.g. a single pulse or two temporally separated pulses at different optical frequencies. The optical source may, in use, exhibit drift in the frequency of the output of the light emitted over time. A change in the base frequency of the output of the optical source will result in a corresponding change in the frequency of the interrogating radiation, with a consequent change in properties of the backscatter interference signal. This change will affect the backscatter interference signal in a similar fashion to a low frequency change in optical path length and represents a noise component in the measurement signal at low frequencies.